Recent developments in automotive technology have introduced various devices to enhance the safety and convenience of the driving experience. For example, air bags are now commonly deployed in vehicles to reduce the likelihood of injury in an accident, and on-board navigation systems are used to help the driver find his/her way to a destination. Another recent development is an on-board telematics system, such as the “OnStar” system. This type of telematics system enables the driver to access many types of services via a wireless communication from the vehicle to an affiliated call center. The call center can then respond to a driver's request for service, which typically includes navigation instructions, roadside assistance, emergency services, and other types of information.
In general, a telematics system is configured as an electronic module installed in a vehicle, and connected to a primary power source, which is typically the main vehicle battery. This type of system is generally intended to provide a driver with various types of call center services, and also to provide an automatic notification capability to a call center in the event of an emergency situation, such as the deployment of an air bag in the vehicle. Therefore, it is generally desirable that a back-up power source be available for the electronic module in the event of primary power disruption. The back-up power source is generally in the form of a relatively low power battery, designed to provide sufficient power to the electronic module to accommodate an emergency situation.
Over lengthy time periods, however, a back-up battery is typically subject to a gradual loss of power capability, even when it is not used to power an electronic module. For example, a typical battery will experience a nominal self-discharge rate (shelf life), and may also experience loss of charge due to various types of leakage currents when connected to any type of electronic circuitry. Therefore, it is desirable to monitor the state-of-charge of a back-up battery in order to implement a timely replacement if the state-of-charge falls below a predetermined threshold level.
Various techniques can be used to anticipate the state-of-charge replacement/threshold level, such as estimation calculations or periodic test sampling. Typically, a simple estimation calculation is based primarily on the nominal self-discharge rate, or shelf life, of the battery. However, this type of calculation may not take other factors into consideration, such as temperature changes and miscellaneous current drains. For example, according to the Arrhenius rate law, chemical reaction rates rise exponentially with reagent temperature. As such, a battery self-discharge rate would be related to temperature.
Another technique involves the periodic sampling of battery voltage, but this intrusive type of testing typically causes current drains from the back-up battery, which can further shorten the life of the battery. Moreover, this technique is generally not a reliable measure of state-of-charge because battery cells typically have a flat voltage curve until nearly discharged.
Accordingly, it is desirable to provide a method of predicting a replacement threshold level for a back-up battery that is minimally intrusive, and that adjusts the predicted self-discharge rate of the back-up battery in accordance with measured temperature values. In addition, it is desirable to provide a prediction method that approximates temperature values during periods when actual temperature measurements cannot be made, such as, for example, when the electronic module is turned off. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.